With known slip control methods, the wheel rotation behavior is measured and used to determine wheel slip and brake pressure modulation. With wheel-specific control (individual control, single wheel control) of vehicle wheels, the brake pressure of each wheel is controlled independently of the rotational behavior of the other wheels. With this approach, it is true that small brake travels may be achieved, but for example braking on highways with different coefficients of friction (referred to as a situation) can result in a yaw torque about a vertical vehicle axis.
That is to say that if the individual vehicle wheels of a vehicle are fully controlled according to slip criteria by means of a slip control system thereof, i.e., according to the control strategy of the slip control system, large pressure differences occur very rapidly between the vehicle wheels on the high coefficient of friction side (referred to below as HM-wheels) and the vehicle wheels on the low coefficient of friction side (referred to below as LM-wheels). The pressure differences (or the longitudinal force differences) cause the yaw torque, which results in a rotation of the vehicle towards the high coefficient of friction side, which cannot be controlled by the driver in general. In order to limit the yaw torque, the wheel pressure on the HM-wheel must be limited in a suitable way.
For example, DE 42 25 983 A1 discloses a method in which the brake pressure at the wheels of an axle is reduced to reduce a yaw torque produced by an ABS control system in such a way that the difference between the brake pressures of an axle does not exceed a maximum permissible value. The maximum permissible value is dependent on the vehicle speed and the lateral acceleration.
Further, it is known to change the ABS control strategy in a μ-split situation according to the so-called “select-low principle”, with which the two rear wheels are controlled to prevent locking depending on the respective vehicle wheel that is currently being operated with the lowest coefficient of friction. This means that in the μ-split situation, the brake of the rear wheel running at the higher coefficient of friction μ is only subjected to the same relatively low brake pressure as the brake of the other rear wheel running at the lower coefficient of friction, although it could also be braked harder without locking because of the higher coefficient of friction prevailing on the HM-wheel.
Further, an ABS control system with GMB (yaw torque limiting) is known from DE 39 25 828 A1, which measures the pressure on the right and left wheels input by the driver to determine the pressure difference. The permissible pressure difference is determined by comparing the target pressure with the actual pressure, and the brake pressure on the other wheel is reduced by means of a control pulse when a predetermined pressure difference is exceeded. The predetermined target pressure difference may be varied depending on the coefficient of friction on the front wheel (LM-wheel) that is running at the low coefficient of friction.
Moreover, DE 41 14 734 A1 also describes an ABS control system with GMB and without pressure sensors. In this case, a value representing the pressure difference at two opposite wheels is continuously determined from pressure reduction signals. In a μ-split-situation, the average pressure build-up gradient on the HM-wheel is varied depending on the pressure difference or the value representing the pressure difference and the deceleration of the vehicle. During the determination of the pressure difference, a weighting is applied that takes into account the profile of the brake system-related pressure reduction gradient that is initially steeper and then flatter with an asymptotic approach to zero.
Both in the cases of DE 39 25 828 A1 and DE 41 14 734 A1, determining the permissible pressure difference is carried out depending on the states of the LM-wheel and the HM-wheel as well as on the driving behavior or on the driving situation of the vehicle.